Hughes's thesis is a synthesis of two separate models for how technology and society interact. One, technological determinism, claims that society itself is modified by the introduction of a new technology in an irreversible and irreparable way--for example, the introduction of the automobile has influenced the manner in which American cities are designed, a change that can clearly be seen when comparing the pre-automobile cities on the East Coast to the post-automobile cities on the West Coast. Technology, under this model, self-propagates as well--there is no turning back once adoption has taken place, and the very existence of the technology means that it will continue to exist in the future.
The other model, social determinism, claims that society itself controls how a technology is used and developed--for example, the rejection of nuclear power technology in the USA amid the public fears after the Three Mile Island incident.
Technological momentum takes the two models and adds time as the unifying factor. In Hughes's theory, when a technology is young, deliberate control over its use and scope is possible and enacted by society. However, as a technology matures, and becomes increasingly enmeshed in the society where it was created, its own deterministic force takes hold. In other words, Hughes's says that the relationship between technology and society always starts with a social determinism model, but evolves into a form of technological determinism over time and as its use becomes more prevalent and important.
Since its introduction by Hughes, the technological momentum concept has been applied by a number of other historians of technology.